DanMa, prior to the advent of the Roth TSP your plan of maxing out your Roth IRA and then adding to the TSP was spot on. Now with your relative low income and low tax liability and the Roth TSP option I recommend people max out their Roth TSP before investing in their Roth IRA. This is due to the lower cost associated with investing in the TSP.
While fees are certainly a consideration, especially in long-term investing, there's an argument to be made in favor of maxing out the Roth IRA before the Roth TSP. First and foremost, obviously, is the vast selection of funds and ETFs you're able to pick from, as opposed to only five that TSP offers. Even if you pay a significantly higher fee (an actively managed fund at the extreme), it could still outperform the comparable TSP fund. Or, at the very least, it could have a better risk-return profile, which is to say that in theory it
should perform better over the long term.
On top of that, Roth TSP requires you to take distributions at 70 1/2 years old, whereas the investments in your Roth IRA could grow tax-free indefinitely, and if you can manage to retire without drawing from it at all (e.g., live off your pension, Social Security, and any TSP money), you could leave it in your will to your kids, significant other, etc. Kind of a morbid benefit, but I digress. My point is that there's something to be said for maxing out either one first. Part of it depends on whether you're a hands-on or hands-off (buy-and-hold) investor.
By the way, a tip for those who don't already know: Morningstar.com has a lot of information, most of it free, on just about any fund you could ever think to own, and more information on those funds than you'll know how to use. If you're handy with Google, you can even find ETF's that track the same indexes as the TSP funds (except for G), and see what Morningstar has to say about them.